A new paleomagnetic study has been carried out on sediments of middle Cambrian age in the North Sichuan Basin (Yangtze Block). Detailed stepwise thermal demagnetizations allowed us to isolate three components. Site-mean direction derived from higher temperature components is D/I=146.9°/–17.1° (95=8.3°) yielding a pole position at 51.3°S, 166.0°E. The fold and reversal tests suggest that remanence was acquired during early stage of sedimentation. Combined with the high-qualities early Sinian (748 Ma) and middle Silurian poles obtained recently from the Yangtze block, the deriving polar track demonstrates a similar loop to that of Australia. After rotating these poles from South China to fit that of Australia, the South China Block is placed against northwestern Australia. This reconstruction favors the correlations of the Jiangnan Grenville-age orogenic belt with the Rudall belt of western Australia, and subsequently the late Proterozoic Jiangnan and Officer/Adelaide rift systems. The paleobiogeographic evidence also indicates that this configuration might maintain by the middle Devonian. 相似文献
The northernmost Kamchatka Peninsula is located along the northwestern margin of the Bering Sea and consists of complexly deformed accreted terranes. Progressing inland from the northwestern Bering Sea, the Olyutorskiy, Ukelayat and Koryak superterranes (OLY, UKL and KOR) are crossed. These terranes were accreted to the backstop Okhotsk-Chukotsk volcanic-plutonic belt (OChVB) in northernmost Kamchatka. A sedimentary sequence of Albian to Maastrichtian age overlaps the terranes and units of the Koryak superterrane, and constrains their accretion time. A paleomagnetic study of blocks within the Kuyul (KUY) terrane of the Koryak superterrane was completed at two localities (Camp 2: λ=61.83°N, φ=165.83°E and Camp 3: λ=61.67°N, φ=164.75°E). At both localities, paleomagnetic samples were collected from Late Triassic (225–208 Ma) limestone blocks (2–10 m in outcrop height) within a melange zone. Although weak in remanent magnetization, two components of remanent magnetization were observed during stepwise thermal demagnetization at 32 sites. The A component of magnetization was observed between room temperature and approximately 250 °C. This magnetic component is always of downward directed inclination and shows the best grouping at relatively low degrees of unfolding. Using McFadden–Reid inclination-only statistics and averaging all site means, the resulting A component mean is Iopt=60.3°, I95=5.0° and n=36 (sites). The B magnetic component is observed up to 565 °C, at which temperature, most samples have no measurable remanent magnetization, or growth of magnetic minerals has disrupted the thermal demagnetization process. Combining sites with Fisher estimates of kappa (k-value)≥13 and n (sites)≥3, where bedding orientation differs within a block, most of these sites show the best grouping of B component directions at 100% unfolding, and two of the blocks display remanent magnetizations of both upward and downward directed magnetic inclination. Combining sites with Fisher estimates of kappa (k-value)≥13 and n (sites)≥3, the resulting overall B component paleolatitude and associated uncertainty are λobs=30.4°N or S, λ95=8.9° and n=19 (sites). When compared with the expected North America paleolatitude of λAPWP expected=57.9°N, our data support a model in which blocks within the Koryak superterrane are allochthonous and far travelled. 相似文献
Paleomagnetic data and rock magnetic results suggest that a widespread orogenic remagnetization caused a pervasive chemical remanent magnetization (CRM) that resides in magnetite in the Mississippian Madison Group in the Sawtooth Range, Montana, during the Late Cretaceous to Early Tertiary. The CRM is similar to a CRM reported by workers from equivalent units in the southern Canadian Cordillera. The CRM is interpreted to be related to alteration by fluids, and there are two likely fluids: hydrocarbons which migrated into the unit and externally derived radiogenic fluids. 相似文献